Polycrystalline superhard materials, such as polycrystalline diamond (PCD) and polycrystalline cubic boron nitride (PCBN) may be used in a wide variety of tools for cutting, machining, drilling or degrading hard or abrasive materials such as rock, metal, ceramics, composites and wood-containing materials. In particular, tool inserts in the form of cutting elements comprising PCD material are widely used in drill bits for boring into the earth to extract oil or gas. The working life of superhard tool inserts may be limited by fracture of the superhard material, including by spalling and chipping, or by wear of the tool insert.
Cutting elements such as those for use in rock drill bits or other cutting tools typically have a body in the form of a substrate which has an interface end/surface and a super hard material which forms a cutting layer bonded to the interface surface of the substrate by, for example, a sintering process. The substrate is generally formed of a tungsten carbide-cobalt alloy, sometimes referred to as cemented tungsten carbide and the super hard material layer is typically polycrystalline diamond (PCD), polycrystalline cubic boron nitride (PCBN) or a thermally stable product TSP material such as thermally stable polycrystalline diamond.
Polycrystalline diamond (PCD) is an example of a superhard material (also called a superabrasive material) comprising a mass of substantially inter-grown diamond grains, forming a skeletal mass defining interstices between the diamond grains. PCD material typically comprises at least about 80 volume % of diamond and is conventionally made by subjecting an aggregated mass of diamond grains to an super high pressure of greater than about 5 GPa, typically about 5.5 GPa and temperature of at least about 1,200° C., for example typically about 1440° C.
PCD is typically formed in the presence of a sintering aid such as cobalt, which promotes the inter-growth of diamond grains. Suitable sintering aids for PCD are also commonly referred to as a solvent-catalyst material for diamond, owing to their function of dissolving, to some extent, the diamond and catalysing its re-precipitation. A solvent-catalyst for diamond is understood be a material that is capable of promoting the growth of diamond or the direct diamond-to-diamond inter-growth between diamond grains at a pressure and temperature condition at which diamond is thermodynamically stable. Consequently the interstices within the sintered PCD product may be wholly or partially filled with residual solvent-catalyst material.
Catalyst materials for diamond typically include any Group VIII element and common examples are cobalt, iron, nickel and certain alloys including alloys of any of these elements. Most typically, PCD is formed on a cobalt-cemented tungsten carbide substrate, which provides a source of cobalt solvent-catalyst for the PCD. Materials that do not promote substantial coherent intergrowth between the diamond grains may themselves form strong bonds with diamond grains, but are not suitable solvent-catalysts for PCD sintering.
During sintering of the body of PCD material, a constituent of the cemented-carbide substrate, such as cobalt in the case of a cobalt-cemented tungsten carbide substrate, liquefies and sweeps from a region adjacent the volume of diamond particles into interstitial regions between the diamond particles.
In this example, the cobalt acts as a catalyst to facilitate the formation of bonded diamond grains. Optionally, a metal-solvent catalyst may be mixed with diamond particles prior to subjecting the diamond particles and substrate to the HPHT process. The interstices within PCD material may at least partly be filled with the catalyst material. The intergrown diamond structure therefore comprises original diamond grains as well as a newly precipitated or re-grown diamond phase, which bridges the original grains. In the final sintered structure, catalyst/solvent material generally remains present within at least some of the interstices that exist between the sintered diamond grains.
Cemented tungsten carbide which may be used to form a suitable substrate is formed from carbide particles being dispersed in a cobalt matrix by mixing tungsten carbide particles/grains and cobalt together then heating to solidify.
To form the cutting element with an super hard material layer such as PCD or PCBN, diamond particles or grains or CBN grains are placed adjacent the cemented tungsten carbide body in a refractory metal enclosure such as a niobium enclosure and are subjected to high pressure and high temperature so that inter-grain bonding between the diamond grains or CBN grains occurs, forming a polycrystalline super hard diamond or polycrystalline CBN layer.
In some instances, the substrate may be fully cured prior to attachment to the super hard material layer whereas in other cases, the substrate may be green, that is, not fully cured. In the latter case, the substrate may fully cure during the HTHP sintering process. The substrate may be in powder form and may solidify during the sintering process used to sinter the super hard material layer.
Ever increasing drives for improved productivity in the earth boring field place create ever increasing demands on the materials used for cutting rock. Specifically, PCD materials with improved abrasion and impact resistance are required to achieve faster cut rates and longer tool life.
Cutting elements for use in rock drilling and other operations require high abrasion resistance and impact resistance. One of the factors limiting the success of the polycrystalline diamond (PCD) abrasive cutters is the generation of heat due to friction between the PCD and the work material. This heat causes the thermal degradation of the diamond layer. The thermal degradation increases the wear rate of the cutter through increased cracking and spalling of the PCD layer as well as back conversion of the diamond to graphite causing increased abrasive wear.
A problem known to exist with such conventional PCD compacts is that they are vulnerable to thermal degradation when exposed to elevated temperatures during cutting and/or wear applications. It is believed that this is due, at least in part, to the presence of residual solvent/catalyst material in the microstructural interstices which, due to the differential that exists between the thermal expansion characteristics of the interstitial solvent metal catalyst material and the thermal expansion characteristics of the intercrystalline bonded diamond, is thought to have a detrimental effect on the performance of the PCD compact at high temperatures. Such differential thermal expansion is known to occur at temperatures of about 400 [deg.] C., and is believed to cause ruptures to occur in the diamond-to-diamond bonding, and eventually result in the formation of cracks and chips in the PCD structure. The chipping or cracking in the PCD table may degrade the mechanical properties of the cutting element or lead to failure of the cutting element during drilling or cutting operations thereby rendering the PCD structure unsuitable for further use.
Another form of thermal degradation known to exist with conventional PCD materials is one that is also believed to be related to the presence of the solvent metal catalyst in the interstitial regions and the adherence of the solvent metal catalyst to the diamond crystals. Specifically, at high temperatures, diamond grains may undergo a chemical breakdown or back-conversion with the solvent/catalyst. At extremely high temperatures, the solvent metal catalyst is believed to cause an undesired catalyzed phase transformation in diamond such that portions of diamond grains may transform to carbon monoxide, carbon dioxide, graphite, or combinations thereof, thereby degrading the mechanical properties of the PCD material and limiting practical use of the PCD material to about 750 [deg.] C.
Attempts at addressing such unwanted forms of thermal degradation in conventional PCD materials are known in the art. Generally, these attempts have focused on the formation of a PCD body having an improved degree of thermal stability when compared to the conventional PCD materials discussed above. One known technique of producing a PCD body having improved thermal stability involves, after forming the PCD body, removing all or a portion of the solvent catalyst material therefrom using, for example, chemical leaching. Removal of the catalyst/binder from the diamond lattice structure renders the polycrystalline diamond layer more heat resistant.
Conventional chemical leaching techniques often involve the use of highly concentrated, toxic, and/or corrosive solutions, such as aqua regia and mixtures including hydrofluoric acid (HF), to dissolve and remove metallic-solvent/catalysts from polycrystalline diamond materials. As such mixtures are highly toxic, the use of these carry severe health and safety risks and therefore processes for treating PCD with such mixtures must be carried out by specialised personnel under well-controlled and monitored conditions to minimise the risk of injury to the operators of such processes.
Furthermore, as PCD material is made more wear resistant, for example by removal of the residual catalyst material from interstices in the diamond matrix, it typically becomes more brittle and prone to fracture and therefore tends to have compromised or reduced resistance to spalling. This is problematic because due to the hostile environment in which cutting elements typically operate, cutting elements having cutting layers with improved abrasion resistance, strength and fracture toughness are desired.
In some conventional PCD material, to improve thermal stability, the interstices are at least partly filled with ceramic material such as SiC or salt material such as carbonate compounds which may be thermally stable.
In other conventional examples, the super hard materials may be formed from composite materials comprising diamond or cBN grains held together by a matrix comprising ceramic material, such as silicon carbide (SiC), or cemented carbide material, such as Co-bonded WC material (for example, as described in U.S. Pat. Nos. 5,453,105 or 6,919,040). For example, certain SiC-bonded diamond materials may comprise at least about 30 volume percent diamond grains dispersed in a SiC matrix (which may contain a minor amount of Si in a form other than SiC). Examples of SiC-bonded diamond materials are described in U.S. Pat. Nos. 7,008,672; 6,709,747; 6,179,886; 6,447,852; and International Application publication number WO2009/013713).
An additional conventional approach to addressing thermal stability in PCD material is to include carbide forming additions such as vanadium carbide (VC) or Co3Sn3C.
Each of the above conventional solutions has various problems associated with it. In particular, the mechanical properties of PCD sintered with cobalt is markedly superior to any other binder system and additions to the cobalt in the form of carbide formers reduce the mechanical properties of the PCD material.
There is a therefore a need to overcome or substantially ameliorate the above-mentioned problems to provide a composite material having increased resistance to spalling and chipping and a method of forming such composites.